Calcitonin-negative medullary thyroid carcinoma: the 'triple-negative' phenotype

Calcitonin-negative medullary thyroid carcinoma is a rare, poorly understood primary neuroendocrine carcinoma of the thyroid characterised by classic medullary thyroid carcinoma morphology without raised serum calcitonin. A 24-year-old woman presented with a slow-growing, right-sided neck swelling. She underwent an ultrasound scan, cytopathological and histopathological examination, and tests for alternative diagnoses. The ultrasound showed a heterogeneous, hyperechoic nodule in the right thyroid lobe. Serum calcitonin was normal. Cytopathology and histopathology showed typical medullary thyroid carcinoma morphology but without calcitonin upon immunostaining and mRNA in situ hybridisation. A 'triple-negative' calcitonin-negative medullary thyroid carcinoma was diagnosed. A completion thyroidectomy with bilateral central lymph node dissection was performed. The patient remains well three-years post-surgery. When cytopathology suggests a medullary thyroid carcinoma, serum calcitonin, pro-calcitonin, carcinoembryonic antigen and calcitonin-gene-related peptide should be measured to identify cases of calcitonin-negative medullary thyroid carcinoma. They should also be measured post-treatment for monitoring purposes. This will aid future calcitonin-negative medullary thyroid carcinoma diagnoses and will inform prognostic stratification and influence treatment decisions.